The biggest question about aliens isn't whether they'll overwhelm us with their superior technology, and subdue the human population. The biggest question is: Why? What could they possibly want from us that they can't get anywhere else? Most of the resources on Earth are available in space, where they're easier to get at, without any pesky gravity well. But the biggest resource that aliens probably couldn't find anywhere but on Earth is... us.

Here are 10 parts of your body that aliens might find it worthwhile to come to Earth and plunder.

Top image: Milla Jovovich in The Fourth Kind.

10. Hair

This tops this list because, honestly, let's hope they're after us for our hair. It's renewable, it requires a certain level of health to be at its best quality — and we don't die when it's harvested. Let's take a look at the facts. Most people agree that mammals were one comet shy of never getting big at all. Who said that anything like us has to exist on other planets? Anyone see any hair on ET, or the creatures from Signs? Your hair keeps forever, but can be reprocessed to provide amino acids for foods like soy sauce and pizza dough. Hair sops up oil at a phenomenal rate. It can be woven together as durable cloth, or placed on farm land as special fertilizer. It even has special optical properties. We could be conquered by terrifying aliens and left expecting the worst — only to be shorn like sheep every year.

9. Poop

This one is slightly less pleasant than hair because we might have to come along with the poop. That would not be a pleasant experience. It's also a long shot. Most species produce their own excrement, and don't need to harvest that of other species, but it's possible that the aliens take in and emit a series of gases, and if that's the case, they might need someone else's poop. Why? Space is full of high-energy radiation that shoots through metal. The contents of a space ship can be shielded by a high electromagnetic field, but that raises other major problems. Layers and layers of dense organic material, on the other hand, absorb a lot of that radiation.

Present-day space agencies already have considered the idea that astronauts on long-haul voyages could line the walls of their spaceships with their food on the way to wherever they're going, and line the walls with their poop (properly packaged, of course) on the way back. Such agencies also mention that it could, with enough work, be possible to process that poop until it become edible again. It's then possible to feed those processed patties to the astronauts, who will then poop them out, and then it's back to the wall. I think at this point it's easy to see why we would rather have the aliens take our hair.

8. Tooth Enamel

When you travel through space, you want to keep your energy down and you want to keep your visibility up. You have to do both of these with material that's either self-repairing or extremely tough. Tooth enamel is the latter. Why would it help any creatures that are floating through the galaxy? Because it glows a bright fluorescent green under UV light. I'm not saying that it's the most important thing in an alien's ship. However, marking out instruments with tooth enamel will be an easy way to read when traversing the skies. So don't be surprised when aliens show up and start pulling out your teeth, and then leave you toothless but alive.

7. Hormones

There's no guarantee that life on other planets will be that different from life on Earth. There are theories that life on this planet emerged from microbial life that came here as debris from other planets. That means that we could, growing from the same building blocks and perhaps exposed to similar conditions, have certain things in common. On this planet we harvest things like hormones for birth control from other creatures. Maybe someday aliens will find that our human hormones prevent them from releasing their eggs into their ancestral spawning waters. Or that, when they latch on to the heads of other creatures and stick tentacles down their throats, they only inject them with a parasite if they feel ready to make a commitment.

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6. Fat

We, as a species, have plenty. And no matter what anyone thinks of it on us, it's an energy source. Perhaps their ships are biological. There is one Earthling who uses it to run his cars. Perhaps, again, the aliens have a biological make-up close to ours and our fat is the best to use in their cosmetics (species have been wiped out for less). Maybe it just makes us taste good. Whatever the reason, you could find yourself getting slimmed down in an unpleasant way.

5. Reproductive Systems

Look, imagine if you had some useful machines. And you saw that, in those useful machines were parts that would make massive quantities of new useful machines, wouldn't you stock up on those parts before you go home?

4. Water

But wait, you're saying. There's tons of water on Earth, easy to collect everywhere. But if you come over from Mars and your one thought is to save your dying planet, are you going back without every drop of water you can squeeze out of your annoying blue neighbor? No! Humans are up to sixty percent water, and there are seven billion of us. And we're very portable. After they suck up all the water from the oceans and rivers and ice caps and the plants, they very well might go ahead and wring out people like sponges. There is, however, a condition on this. The universe actually has a lot of water, but the largest quantities of it tend to be far away, out of the solar system. Only planets within our own solar system would see this kind of thing as worthwhile. So if it turns out there aren't any advanced Martians or Venusians, we're off the hook for this one!

3. Immortal Cell Lines

Our cells are, mostly, subject to the Hayflick Limit, which sets a cap on how many times that cells can reproduce. This cruelly limited cell biologists, until they stumbled on to cancer cells from Henrietta Lacks. Her cells (called HeLa cells) kept dividing and dividing, making them perfect for use in experiments. Since then, scientists have found ways to induce immortality in cell lines — for humans. Aliens might not be subject to the same kinds of limits that human cells are. There may be no way to induce that kind of immortality, or they may not have discovered the difference between immortal and mortal cells at all. In this case, these cells might serve the purpose that guiding animal cells did before HeLa cells were found. Even if they know perfectly well what will work for them, there's a reason why HeLa and other immortal cell lines are in laboratories the world over. They are vital to biological research, and so they'd be vital to any species that wanted human biological tools.

2. Arms

Who knows what kind of things will come over to Earth through the barren wasteland of space? It's likely that if they're mechanically apt they manage to make it to Earth, but then again, who knows what they've transcended in order to make the journey. The human hand, however, has managed to paint masterpieces, done the lifting that built cathedrals, and carefully put together machines that, in turn, put together even more machines. It's an amazingly good mechanical object, and though it takes some time to get the hang of, it has a flexibility, dexterity, and strength. Go ahead and try not to think of alien ships, or homes, lined with helpful human arms.

1. Brains

Oh, like I was going to pick anything else for number one? There are a lot of good teeth out there. There's a lot of fat and poop and hair in the animal kingdom. But the human brain is, not to be too speciesist about it, the state of the art brain on planet Earth. Able to maintain long-term networks. Able to adapt them when they're destroyed. Able to make new cells, to focus and learn from almost anything. And with a capacity for symoblic and linguistic thought that isn't matched anywhere else in the animal kingdom, this is the stuff right here. We talk a lot about training computers to learn, or think, like brains. What if the aliens that finally make it to Earth have given that up and taught brains to work like computers? If that was the case, each new brain they picked up throughout the universe, is the equivalent of picking up whole new parts with new functions and new specializations. Since the human brain has a lot of unique features, but it also adaptable to new situations with a little training, if the aliens are going to focus on humans at all, they'll focus on us for us brains. Human minds might travel the universe more extensively as part of alien space ships than in our own bodies.

All things considered, though, I'd prefer it if they just came for hair.